Arsenic residues in pastures and horses after MSMA-based herbicide control of Urochloa decumbens

Abstract The diet of horses is primarily based on grasses such as Coast Cross (Cynodon spp.), with Urochloa decumbens (formerly Brachiaria decumbens) being the main invasive species. The control of U. decumbens is typically performed using the herbicide MSMA (monosodium methylarsonate), which degrades into four main arsenic species after application: arsenite, arsenate, monomethylarsonic acid, and dimethylarsinic acid. This study aimed to determine whether the spray of MSMA is a determining factor in increasing arsenic concentrations in the environment and in horses. Arsenic levels in soil, plants, and animals were compared following herbicide use. Analytical results revealed higher arsenic accumulation in U. decumbens (26.20 ± 0.08 ppm) compared to Coast Cross (12.66 ± 2.94 ppm) (p < 0.0001). U.decumbens accumulated 10.69 ± 1.69 ppm in the MSMA-treated area versus 6.06 ± 0.96 ppm in the control area (p < 0.0001). Coast Cross showed arsenic concentrations of 10.73 ± 1.22 ppm in the MSMA-treated area and 1.60 ± 0.41 ppm in the control area (p < 0.0001). Horses grazing on MSMA-treated pastures presented significantly higher arsenic levels in blood (1.67 ± 0.047 ppm) compared to the control group (1.35 ± 0.0531 ppm) (p < 0.05). In urine, arsenic levels were 1.62 ± 0.189 ppm in the MSMA group and 1.195 ± 0.111 ppm in the control group (p < 0.05). The results demonstrate that MSMA sprayed in equine pastures leads to significant arsenic accumulation in forage plants and in horses, representing a potential risk to animal health and food safety.